External leakage of gases from a fuel cell stack is either due to cracks in ceramic components, failed seals or gas transport through porous layers.
In a fuel cell stack comprising a number of cells, each cell comprises a gas tight and oxygen ion conducting electrolyte and a porous cathode and a porous anode. The reactants can diffuse through the porous layers so the reaction can take place. An electrochemical process takes place when hydrogen is transported to the anode and oxygen is transported to the cathode thereby allowing oxygen ions from the cathode to be transported via the electrolyte to the anode whereby hydrogen on the anode reacts with the oxygen in this way creating H2O and a transport of electrons.
Porous anodes and cathodes can be prepared from ceramic powder. Also porous supports for e.g. anodes are known, where the porous supports are based on powder metallurgy. An example of a porous support is e.g. described in EP patent application No. 1122806 in which the gas supply channels with the cell support and the anode layer are integrated. Since the support is porous, part of the gas diffuses through the support to the electrolyte while another part of the gas is transported to the rim of the support. In order to avoid leakage to the surroundings the rim is gas impermeable.
A gas impermeable rim can be obtained by using glass as a sealing material as disclosed in EP patent application No. 1010675. A drawback of using glass is that it is brittle and that its thermal properties are quite different from the thermal properties of metal, which may cause difficulties when the stack is heated or cooled during operation.
Furthermore, due to their high viscosities, glass melts are difficult to impregnate into the porous layer.
WO 2007080518 describes a fuel cell unit which on the outside is sealed by injection moulding.
It is an object of the invention to provide a unit for a fuel cell stack in which gas leakage to the surroundings is prevented.
It is another object to provide an alternative way of reducing the number of elements in a fuel cell stack.
It is a further object to provide a fuel cell that is more robust to thermal stress during use.